Circuit topology for pulsed power energy harvesting

ABSTRACT

An energy harvesting circuit harvests energy from a voltage source and charges a storage element with the harvested energy. The energy harvesting circuit includes an energy source, a storage capacitor to store energy output from the energy source, a power converter circuit, an energy storage element, and an enabling circuit. The enabling circuit turns the boost converter circuit on and off according to a monitored capacitance voltage of the storage capacitor. When the boost converter circuit is turned off, the storage capacitor accumulates energy output from the energy source until a reference voltage is reached, whereupon the boost converter circuit is turned on, enabling current flow from the storage capacitor to the storage element. When the storage capacitor discharges to a minimum voltage level, the boost converter circuit is turned off. The enabling circuit and a reference voltage supply are powered by the energy source.

RELATED APPLICATIONS

This application claims priority of U.S. provisional application, Ser.No. 61/319,169, filed Mar. 30, 2010, and entitled “NEW CIRCUIT TOPOLOGYFOR PULSED POWER ENERGY HARVESTING”, by the same inventor. Thisapplication incorporates U.S. provisional application, Ser. No.61/319,169 in its entirety by reference.

FIELD OF THE INVENTION

The present invention relates to the field of energy harvesting. Moreparticularly, the present invention relates to the field of low powerenergy harvesting to charge energy storage devices.

BACKGROUND OF THE INVENTION

An energy harvesting circuit can be used to harvest energy from anenergy source and charge a battery using the harvested energy. There areconventional energy harvesting circuits configured to harvest energy atvery low power levels if the energy source voltage is the same or higherthan the battery voltage. There are also known energy harvestingconfigurations for harvesting energy at levels down to about 50 uW whenthe harvesting source voltage is 8 to 20 times lower than the output(battery) voltage. Current needs for energy harvesting are at or below afew microwatts, for example about 2 uW, and trending downward. Smallersized energy sources generate low power levels at low voltage levels. Anexample of such an energy source is a small solar cell that generatesabout 2 uW and about 0.5V. A problem lies in how to harvest energy at avoltage level that is much less than the battery voltage, and to boostthe harvested voltage to the battery voltage in an efficient means atthose power levels.

FIG. 1A illustrates a conceptual schematic diagram of a conventionalenergy harvesting circuit configured to harvest energy from a low powersource at low voltage to charge a battery. The energy harvesting circuit10 includes a low voltage source 20, a storage capacitor C1, an inductorL1, a transistor T1, a diode D1, a capacitor C2, a boost controllercircuit 30, a battery 40, a comparator 50, a resistor R1 and a resistorR2. The boost controller circuit 30, the transistor T1, the inductor L1,the diode DL and the capacitor C2 form a boost converter circuit. Thelow voltage source 20 is a low level power source, conceptuallyrepresented as voltage source 22. In an exemplary application, the lowvoltage source 20 generates 2 uW at 0.5V. The low voltage source 20 alsohas a source impedance, conceptually represented as resistor RS. As usedherein, reference to a “source voltage” refers to voltage VS across thevoltage source 22. The transistor T1 functions as a switch that enablescurrent flow from the storage capacitor C1 to the inductor L1 and thento the battery 40, this configuration is typically called a boostconverter. When the transistor T1 is turned on, the voltage at VC isapplied across the inductor L1 allowing energy to be stored in theinductor L1. While the transistor T1 is on, the diode D1 is reversebiased, thereby blocking the battery 40 voltage and not allowing currentto flow out of the battery 40. When the transistor T1 is turned off, thestored energy in the inductor L1 flows through the diode D1 and deliversenergy into the battery 40. The capacitor C2 is in parallel with thebattery 40 and is used to reduce the impedance of the battery 40 at theswitching frequency, and thus filter out pulses of energy coming fromthe diode D1.

The boost controller circuit 30 supplies a control signal as a gatevoltage to the transistor T1, thereby turning the transistor T1 on andoff. The boost controller circuit 30 provides a Pulse Width Modulated(PWM) signal to the gate of the transistor T1, thus modulating theamount of energy that is delivered to the battery 40. In the exemplaryenergy harvesting circuit of FIG. 1A using this type of boostcontroller, the duty cycle is fixed and the output of the boostcontroller is regulated by means of a Pulsed Frequency Modulation (PFM)input. The PFM signal regulates the output within a voltage window thatis set by an amount of hysteresis in comparator 50 and the reference(REF) input. In other examples, the duty cycle of the PWM output iscontrolled by a circuit inside of the boost controller circuit 30 thatchanges the duty cycle as a function of the output voltage when comparedto an internal reference. In the example of FIG. 1A, the boostcontroller circuit 30 is turned on and off by the PFM Enable signalsupplied by the comparator 50. A first input of the comparator 50 iscoupled to an output of the battery 40. A second input of the comparator50 is coupled to a reference voltage. The output of the comparator 50goes low when the battery voltage is above the reference voltage andthis is set to be at the fully charged battery voltage. The output ofthe comparator 50 does not go high until the battery voltage is reducedto a minimum voltage level that is less than the reference voltage dueto hysteresis built into the comparator 50. For battery charging, if thebattery 40 is at the regulation voltage, then there would be no need forharvesting since the battery would already be charged.

The energy harvesting circuit 10 uses pulse frequency modulation (PFM)to harvest the low power level generated by the low voltage source 20.This is accomplished by monitoring the boosted output voltage across thebattery 40 with the comparator 50. However, this type of converterachieves low power operation only when the output is at its desiredregulation level and this is when the battery is fully charged. In orderto charge the battery 40, the boost converter of FIG. 1A consumes toomuch power and can not achieve harvesting at 2 uW level. Whenever thebattery 40 is lower that the preset voltage level (fully charged)determined by the resistors R1, R2 and the reference voltage (REF), theoutput of the comparator 50 remains high allowing the boost controller30 to run continuously. For a battery that is below the regulation leveland needs to be charged, the boost controller 30 of FIG. 1A attempts todeliver more energy than there is available at the low voltage source20, thereby dragging down the voltage at VC. Since the boost controller30 requires supply current that comes from the battery 40 to perform itsfunction, and if the voltage at VC is too low to allow sufficient energyto be replaced, the boost converter would remove more energy from thebattery than it would deliver into the battery.

A disadvantage of the energy harvesting circuit 10 is that in order toharvest energy of a few microwatts at low voltage, it is necessary toconsume less energy out of the battery to operate the boost controller30 compared to the amount of energy being delivered back into thebattery. Even assuming that the supply current problem can be overcome,there are other problems with the energy harvesting circuit of FIG. 1A.In order to harvest low power at the 2 uW level, the boost controller 30needs to have the average inductor L1 current be equal to the averagecurrent available from the low voltage source 20. One way to achievethis low current is to make the inductor L1 very large, which isundesirable for most applications. Another way to make the averageinductor current very low is to make the converter operate at a very lowduty cycle, but this limits the boost converter to one power level andincreases the supply current. In an exemplary application, the lowvoltage source 20 generates 2 uW at 0.5V, and the battery 40 has acharged voltage of 4V. To harvest 2 uW at 0.5V the supply current fromthe 4V battery to the boost controller 30 needs to be about 100 nA, thedrain to source capacitance of the transistor T1 needs to be about 0.1pF, and the impedance of the inductor L1 needs to be about 40 mH.Typical supply currents for boost converters are in the 10 uA to 100 uArange so it is unrealistic to harvest energy below 40 uW.

The comparator 50 monitors the output of the battery 40, and if thebattery voltage is greater than a reference voltage, the PFM Enablesignal is low and the boost converter circuit is off. Even with theboost converter circuit off, the comparator 50 still requires power tooperate and perform the comparison function. Although the energyharvesting circuit 10 provides a pulse modulation means for periodicallyturning on and off the boost converter circuit, this architecture isineffective for charging the battery 40. Using the energy harvestingcircuit 10, if the battery 40 needs to be charged, for example themonitored battery output is less than the reference voltage, then theboost controller circuit 30 will always be turned on and the boostconverter circuit will always be consuming more power than it candeliver to the battery 40. The low voltage source 20 does not generateenough power to continuously power the boost converter circuit. Theboost converter circuit would only turn on and drag down the voltage VCwhile harvesting, and then consume more power from the battery 40 thanis able to be delivered from the low voltage source 20.

The PFM type method utilized in FIG. 1A is useful for turning the boostconverter circuit on and off, but the energy harvesting circuit is notapplicable to charging a battery. Instead, the PFM type method used inthe energy harvesting circuit 10 is more useful for consuming energyfrom a battery and supplying energy pulses to a load. For example, theenergy harvesting circuit of FIG. 1A is adapted to replace the lowvoltage source 20 with a battery, and replace the battery 40 with a loadthat requires power delivery of only a couple microwatts. In thisconfiguration, the boost converter circuit can deliver only a couplemicrowatts, and shut down when not needed. Although useful for providingenergy bursts to a load, the energy harvesting circuit described in FIG.1A does not provide an effective means for charging a battery.

FIG. 1B illustrates a conceptual schematic diagram of a conventionalenergy harvesting circuit configured to harvest energy from a powersource to charge a battery. The energy harvesting circuit 80 includes apower source 24, a storage capacitor C5, an inductor L3, a diode D3, acapacitor C7, a converter controller circuit 60, a battery 42, acapacitor C6, resistors R3-R10, and an enabling circuit 70. Theconverter controller circuit 60, the inductor L3, the diode D3, thecapacitor C6, and the capacitor C7 form a converter circuit. Energy fromthe solar cell power source 24 is stored in the capacitor C5. Theconverter controller circuit 60 includes similar functionality as theboost controller circuit 30 and the transistor T1 of FIG. 1A to enableor disable current flow from the capacitor C5 to the inductor L3, andfrom the inductor L3 to the battery 42. The converter controller circuit60 is enabled and disabled by the enabling circuit 70, which includes acomparator 72, a comparator 74, and a diode D4. A reference voltage isinput to each of the comparators 72 and 74. The voltage of the capacitorC5 is monitored by the comparator 72 and compared to a referencevoltage. Once the capacitor voltage reaches the reference voltage, thecomparator 72 outputs an enabling signal to the converter controllercircuit 60. This initiates discharging of the capacitor C5 for chargingof the battery 42. Once the capacitor C5 is discharged to apredetermined minimum voltage level, the comparator 72 outputs a disablesignal to the converter controller circuit 60, which in turn is turnedoff thereby stopping discharge of the capacitor C5.

The comparator 74 monitors the battery voltage and disables theconverter controller circuit 60 when the battery voltage reaches a highlimit and enables the converter controller circuit 60 when the batteryvoltage declines below a predetermined level. The dual comparator withhysteresis controls shutdown of the converter controller circuit 60 andalso the selection of two different charging rates, such as a fastcharge and a trickle charge. During fast charge, the convertercontroller circuit 60 operates as a current source, forwarding inductorenergy from the inductor L3 to the battery 42 without checking thebattery voltage. When charging a discharged battery 42, the energyharvesting circuit 80 applies full fast-charge current until the batteryvoltage reaches its upper limit. The converter controller circuit 60 isthen disabled until the battery voltage declines to the next-lowestlimit, whereupon the converter controller circuit 60 is enabled for thetrickle charge. Trickle charging continues until the battery voltagereaches its upper limit, whereupon the converter controller circuit 60is disabled, or its lower limit, whereupon the converter controllercircuit 60 is enabled for fast-charge again.

As shown in FIG. 1B, the enabling circuit 70 is provided supply voltageV+ from the battery 42. Further, the resistor divider R9, R10 is coupledto the battery 42. As such, the enabling circuit 70 and the resistordivider R9, R10 act as loads on the battery when the convertercontroller circuit 60 is turned off, or when the energy harvestingcircuit 80 is not connected to the solar cell power source 24. Althoughthe energy harvesting circuit 80 may be configured for minimal batterydischarge during these conditions, the energy harvesting circuit 80 isineffective for those applications requiring the energy harvestingcircuit 80 to apply zero load to the battery when disabled.

SUMMARY OF THE INVENTION

Embodiments of an energy harvesting circuit are directed to harvestingenergy from a voltage source and charging a storage element with theharvested energy. In some embodiments, the energy harvesting circuit isdirected to harvesting energy from a very low voltage source at very lowpower levels. The energy harvesting circuit includes an energy source, astorage capacitor to store energy output from the energy source, a powerconverter circuit, an energy storage element, and an enabling circuit.In some embodiments, the power converter circuit is a step-up converter,for example a boost converter. In other embodiments, the power convertercircuit is a step-down converter. The enabling circuit turns the powerconverter circuit on and off according to a monitored capacitancevoltage of the storage capacitor. When the power converter circuit isturned off, the storage capacitor accumulates energy output from theenergy source until a reference voltage is reached. While turned off, noload from the energy harvesting circuit draws current from the energystorage element. Once the storage capacitor voltage reaches thereference voltage, the power converter circuit is turned on, enablingcurrent flow from the storage capacitor to the energy storage element.In some embodiments, the energy storage element voltage is greater thanthe energy source voltage and the storage capacitor voltage. The powerconverter circuit enables charging of the energy storage element by anenergy pulse received from the storage capacitor. When the storagecapacitor discharges to a minimum voltage level, the power convertercircuit is turned off. The storage capacitor is recharged by energyoutput from the energy source. The enabling circuit and a referencevoltage supply are both powered by the energy source.

In an aspect, a circuit includes an energy storage element; an energysource; a storage capacitor coupled to the energy source and configuredto store energy output from the energy source; a power converter circuitcoupled between the storage capacitor and the energy storage element,wherein the power converter circuit converts a storage capacitor voltagestored by the storage capacitor to a voltage to charge the energystorage element; and an enabling circuit coupled between the energysource and the power converter circuit, wherein the enabling circuitpulse modulates the power converter circuit to enable burst energytransfer from the storage capacitor to the energy storage elementwhenever the storage capacitor voltage reaches a reference voltage,further wherein the enabling circuit is powered by the energy source.

The enabling circuit can be configured to compare the storage capacitorvoltage to the reference voltage and output an enabling signal to thepower converter circuit if the capacitor voltage is greater than orequal to the reference voltage and output a disabling signal to thepower converter circuit if the capacitor voltage is less than thereference voltage. The power converter circuit can be configured toenable burst energy transfer from the storage capacitor to the energystorage element when turned on by the enabling signal and to disableenergy transfer when turned off by the disabling signal. When the powerconverter circuit is disabled there is no power drain on the energysource due to a load. When the power converter circuit is disabled theonly power drain on the energy source is due to parasitic drain. Theenergy storage element has a storage element voltage and the energysource generates power at a source voltage, in some embodiments thesource voltage is less than the storage element voltage. In an exemplaryapplication, the energy source generates power in the range of about 2uW to about 200 uW, and the energy source generates between about 0.4Vand about 0.8V. The power converter circuit can be a step-up converteror a step-down converter. The energy storage element can be a battery ora super capacitor. The power converter circuit is powered by the energystorage element when the power converter circuit is turned on. The powerconverter circuit can include a converter controller circuit and atransistor coupled to the converter controller circuit, the transistorconfigured to enable and disable current flow between the storagecapacitor and the energy storage element, and the converter controllercircuit is configured to turn on and off the transistor. The circuit canalso include a reference voltage supply coupled to the enabling circuitand configured to supply the reference voltage to the enabling circuit,wherein the reference voltage supply is powered by the energy source.The energy source can be a DC power source or an AC power source. If theenergy source is an AC power source the circuit also includes arectifying circuit coupled between the AC power source and the storagecapacitor. The enabling circuit can have positive feedback.

In another aspect, a circuit includes an energy storage element; anenergy source; a storage capacitor coupled to the energy source andconfigured to store energy output from the energy source; a powerconverter circuit coupled between the storage capacitor and the energystorage element, wherein the power converter enables energy transferfrom the storage capacitor to the energy storage element when turned onby an enabling signal and disables energy transfer when turned off by adisabling signal; and an enabling circuit coupled between the energysource and the power converter circuit, wherein the enabling circuit ispowered by the energy source, further wherein the enabling circuitcompares a storage capacitor voltage to a reference voltage and outputsthe enabling signal to the power converter circuit if the capacitorvoltage is greater than or equal to the reference voltage and outputsthe disabling signal to the power converter circuit if the capacitorvoltage is less than the reference voltage.

In yet another aspect, a circuit includes an energy storage element; anenergy source; a storage capacitor coupled to the energy source andconfigured to store energy output from the energy source; a powerconverter circuit coupled between the storage capacitor and the energystorage element, wherein the power converter enables energy transferfrom the storage capacitor to the energy storage element when turned onby an enabling signal and disables energy transfer when turned off by adisabling signal; an enabling circuit coupled between the energy sourceand the power converter circuit, wherein the enabling circuit is poweredby the energy source, further wherein the enabling circuit compares astorage capacitor voltage to a reference voltage and outputs theenabling signal to the power converter circuit if the capacitor voltageis greater than or equal to the reference voltage; and a disablingcircuit coupled between the energy source and the power convertercircuit, wherein the disabling circuit is powered by the energy storageelement only when the power converter circuit is enabled, furtherwherein the disabling circuit compares the storage capacitor voltage tothe reference voltage and outputs the disabling signal to the powerconverter circuit if the capacitor voltage is less than the referencevoltage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a conceptual schematic diagram of a conventionalenergy harvesting circuit configured to harvest energy from a low powersource at low voltage to charge a battery.

FIG. 1B illustrates a conceptual schematic diagram of anotherconventional energy harvesting circuit configured to harvest energy froma low power source at low voltage to charge a battery.

FIG. 2 illustrates a conceptual schematic diagram of an energyharvesting according to an embodiment.

FIG. 3 illustrates a conceptual schematic diagram of an energyharvesting according to another embodiment.

FIG. 4 illustrates an exemplary capacitor voltage function for thestorage capacitor C3 in FIG. 2.

Embodiments of the energy harvesting circuit are described relative tothe several views of the drawings. Where appropriate, the same referencenumeral will be used to represent the same or similar elements.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present application are directed to an energyharvesting circuit. Those of ordinary skill in the art will realize thatthe following detailed description of the energy harvesting circuit isillustrative only and is not intended to be in any way limiting. Otherembodiments of the energy harvesting circuit will readily suggestthemselves to such skilled persons having the benefit of thisdisclosure.

Reference will now be made in detail to implementations of the energyharvesting circuit as illustrated in the accompanying drawings. The samereference indicators will be used throughout the drawings and thefollowing detailed description to refer to the same or like parts. Inthe interest of clarity, not all of the routine features of theimplementations described herein are shown and described. It will, ofcourse, be appreciated that in the development of any such actualimplementation, numerous implementation-specific decisions must be madein order to achieve the developer's specific goals, such as compliancewith application and business related constraints, and that thesespecific goals will vary from one implementation to another and from onedeveloper to another. Moreover, it will be appreciated that such adevelopment effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking of engineering for those ofordinary skill in the art having the benefit of this disclosure.

Embodiments of an energy harvesting circuit are directed to harvestingenergy from a voltage source and charging a storage element with theharvested energy. The energy harvesting circuit includes an energysource, a storage capacitor to store energy output from the energysource, a power converter circuit, an energy storage element, and anenabling circuit. In some embodiments, the power converter circuit is astep-up converter, for example a boost converter. In other embodiments,the power converter circuit is a step-down converter. Embodiments of theenergy harvesting circuit are described below in terms of the energyharvesting circuit including a boost converter circuit. It is understoodthat the principles described can be applied to alternative types ofstep-up converters, such as a buck-boost converter, or to step-downconverters. In some embodiments, the energy harvesting circuit isdirected to harvesting energy from a very low voltage source at very lowpower levels. In an exemplary application, the low voltage sourceoutputs power in the range of about 2 uW to about 200 uW and at voltagein the range of about 0.4V to about 0.8V.

The energy harvesting circuit pulse modulates a harvesting period toenable concentration of energy into manageable power bursts for chargingthe storage element. Energy generated by the energy source is stored bythe storage capacitor using low current. Once the capacitor voltagereaches a predetermined reference voltage, the boost converter circuitis turned on, thereby discharging the storage capacitor and charging thestorage element. The boost converter circuit boosts the capacitorvoltage to a level that is greater than the storage element voltage.Once the storage capacitor is discharged to a predetermined minimumvoltage level, the boost converter circuit is turned off. The enablingcircuit is coupled to the low voltage source and is powered by a lowcurrent supplied by the low voltage source. The enabling circuit is alsocoupled to the storage capacitor to compare the storage capacitorvoltage to the reference voltage. The reference voltage is provided by areference voltage supply. Both the enabling circuit and the referencevoltage supply are powered by the energy source. If the storagecapacitor voltage is greater than or equal to the reference voltage, anenable signal is sent from the enabling circuit to the boost convertercircuit. The enable signal turns on the boost converter circuit. If thestorage capacitor circuit is less than the reference voltage, a disablesignal is sent from the enabling circuit to the boost converter circuit.The disable signal turns off the boost converter circuit. While theboost converter circuit is turned off, there is no power drain on thestorage element other than parasitic drain. In other words, no load fromthe energy harvesting circuit draws current from the storage elementwhile the boost converter circuit is turned off. The boost convertercircuit does not draw power from the storage element when disabled. Whenenabled, the boost converter circuit is powered by the storage element.

An advantage of the energy harvesting circuit configuration is thatenergy can be harvested from the low voltage source at any power leveldown to the supply current of the enabling circuit. In an exemplaryapplication, the use of an enabling circuit with a supply current of 50nA and a supply voltage of 500 mV results in the ability to harvestenergy down to 25 nW. When enabled, the boost converter circuit ispowered by the storage element. The supply current of the boostconverter circuit need only be less than the output current of the boostconverter circuit to enable efficient pulses of energy to be deliveredfrom the storage capacitor to the storage element when the boostconverter circuit is turned on.

FIG. 2 illustrates a conceptual schematic diagram of an energyharvesting according to an embodiment. The energy harvesting circuitharvests energy from a low voltage source to charge a storage element,such as a battery. In some embodiments, the low voltage source is asolar cell. In other embodiments, the low voltage source is any otherconventional low voltage source including, but not limited to, a thermalelectric generator (TEG). The energy harvesting circuit is configured topulse modulate the transfer of the harvested energy to the battery. Theenergy harvesting circuit 100 includes a low voltage source 120, astorage capacitor C3, an inductor L2, a transistor T2, a diode D2, acapacitor C4, a boost controller circuit 130, a battery 140, an enablingcircuit 150, and a reference voltage supply 170. The low voltage source120 is a low level power source, conceptually represented as voltagesource 122 having a voltage VS. The low voltage source 120 also has asource impedance, conceptually represented as resistor RS. As usedherein, reference to a “source voltage” refers to the voltage VS acrossthe voltage source 122. In an exemplary application, the source voltageis about 0.5V, and the low voltage source 120 generates 2 uW at 0.5V. Avoltage source is typically referred to as low voltage when a harvestingcircuit, such as the boost converter circuit of FIG. 2, is not able tooperate directly from the low voltage source and needs to consume powerfrom a battery or other source that is within the needed voltage level.A low level power source is a source that delivers less power than ittakes to run a voltage converter, such as the boost converter circuit ofFIG. 2, without any power delivered to the load.

The boost controller circuit 130, the transistor T2, the inductor L2,the diode D2, and the capacitor C4 form a boost converter circuit forboosting the voltage stored in the capacitor C3 to a voltage level thatis greater than the battery voltage. The transistor T2 functions as aswitch that enables current flow from the storage capacitor C3 to theinductor L2 and then to the battery 140, this configuration is typicallycalled a boost converter. When the transistor T2 is turned on, thevoltage at VC is applied across the inductor L2 allowing energy to bestored in the inductor L2. While the transistor T2 is on, the diode D2is reverse biased, thereby blocking the battery 140 voltage and notallowing current to flow out of the battery 140. When the transistor T2is turned off, the stored energy in the inductor L2 flows through thediode D2 and delivers energy into the battery 140. The capacitor C4 isin parallel with the battery 140 and is used to reduce the impedance ofthe battery 140 at the switching frequency, and thus filter out pulsesof energy coming from the diode D2. The transistor T2 is turned on andoff by the boost controller 130.

The boost controller 130 is turned on and off with the PFM Enablesignal. The PFM Enable signal is output by the enabling circuit 150. Insome embodiments, the enabling circuit 150 is a hysteretic comparator. Apositive input of the comparator 150 is coupled to the capacitor C3. Anegative input of the comparator 150 is coupled to the reference voltagesupply 170 to receive as input a reference voltage. The comparator 150compares the voltage of the capacitor C3 to the reference voltage. ThePFM Enable signal output of the comparator 150 goes high (enables) whenthe voltage on the capacitor C3 is greater than or equal to thereference voltage. The PFM Enable signal goes low (disables) when thevoltage on the capacitor C3 is lower than a predetermined minimumvoltage level. The PFM Enable signal output from the comparator 150 doesnot go low until the battery voltage is reduced to the minimum voltagelevel due to hysteresis built into the comparator 150. In an exemplaryapplication, the reference voltage is about 0.5V, and the minimumvoltage level is about 0.3V. The voltage levels that dictate the outputof the comparator 150 are only exemplary, alternative voltage levels canbe used.

For the boost converter circuit to work properly in a system that hasthe battery connected to the input supply of the boost converter circuitand at the same time not draw any current when disabled, it requiresthat the enabling input to the boost converter circuit has a CMOSSchmitt trigger input stage that disables everything in the boostconverter circuit. If the input to the enabling pin on the boostconverter circuit is anything but high or low then it will discharge thebattery. If the enabling circuit (comparator 150) has low gain at itstrip point, then there is the possibility to have the output of theenabling circuit be partially on. Any output state that is not fully onor fully off will cause the boost converter circuit input to draw powerand thus discharge the battery. When the enabling circuit has very highgain or hysteresis, sometimes referred to as positive feedback, thenthere is not any voltage level at its input that will allow its outputto be at any level except high or low.

The comparator 150 and the reference voltage supply 170 are powered fromthe low voltage source 120, not from the battery 140. The comparator 150and the reference voltage supply 170 are not powered by the battery 140because if it were connected to the battery 140 and the voltage VCremained less than the reference voltage level for a long period of timeand therefore no energy harvesting, the comparator 150 and the referencevoltage supply 170 would remove some or all stored energy in the battery140. If the comparator 150 and the reference voltage supply 170 areconnected to the low voltage source 120, as is the case in the energyharvesting circuit 100, then with these same conditions of no energyharvesting, this configuration allows the battery 140 to remain at itscurrent storage level and not be discharged by a load. While the boostconverter circuit is turned off, there is no power drain on the battery140 other than parasitic drain. In other words, no load from the energyharvesting circuit 100 draws current from the battery 140 while theboost converter circuit is turned off. The boost converter circuit doesnot draw power from the battery 140 when disabled. When enabled, theboost converter circuit is powered by the battery 140.

The supply current to the comparator 150 and the reference voltagesupply 170 must be low enough to allow the charging of the capacitor C3.To determine the allowable supply current of comparator 150 and thereference voltage supply 170, the power consumed by the comparator 150and the reference voltage supply 170 must be less than the desiredharvest value, where the power consumed by the comparator 150 and thereference voltage supply 170 is the supply current multiplied by thevoltage VC. In the example of 2 uW of energy harvesting at 0.5V, thecomparator 150 and the reference voltage supply 170 need to have asupply current less than 4 uA if connected to the low voltage source120.

In an alternate configuration, the comparator 150 and the referencevoltage supply 170 can be powered by a secondary low voltage source thatcan deliver lower power than the primary power source, but the primarypower source is at a much lower voltage than the secondary power source.An example of the alternate configuration with two input power sourcesis where the primary power source voltage is in the range of about0.025V to about 0.05V, while the secondary power source voltage is inthe range of about 0.4V to about 0.8V.

In another alternative configuration, shown in FIG. 3, a comparator 150′is the same as the comparator 150 (FIG. 2) except that the output of thecomparator 150′ is only used to enable a boost controller circuit 130′and not to disable the boost controller circuit 130′. In thisalternative configuration, a separate comparator 160 is used to disablethe boost controller circuit 130′. The disabling comparator 160 issupplied from the battery 140 because the disabling comparator 160 canbe turned off when the boost controller circuit 130′ is off and thus notdrain the battery 140 when the harvesting source is not present. In someembodiments, the enabling input from comparator 150′ and the disablinginput from comparator 160 are connected to Set and Reset inputs of alatch inside of the boost controller circuit 130′.

Although the enabling circuit is described as a comparator, otherconfigurations are contemplated. In general, the enabling circuit ispowered by the low voltage source, the enabling circuit has trip levelsthat are manageable within some tolerance range that is acceptable, andthe enabling circuit outputs an enabling signal.

Referring to FIG. 2, the boost converter circuit, and in particular theboost controller 130, draws power from the battery 140 when enabled.When turned on, the boost controller 130 uses a certain amount of powerto operate. The transistor T2 has a drain to source capacitance, whichresults in a power loss each time the transistor is switched.Additionally, there are power losses in the inductor L2 and the diodeD2. As such, there is a minimum power requirement to operate the boostconverter circuit. The energy harvesting circuit 100 is configured toharvest energy in bursts at high power levels for very short periods oftime. For example, 2 uW of power can be harvested with a boost converterconnected to a 4V battery, the boost converter having a supply currentof 10 uA and operated in a pulsed mode of operation. In this example,the boost converter consumes 40 uW of power from the battery whiledelivering 10 mW of power into the battery, which is highly efficient.In order to balance the available input power of 2 uW to the 10 mW pulseof higher power, this example using the energy harvesting circuit 100enables the 10 mW to charge the battery for about 20 u seconds and thenturn off for about 100 m seconds. The value of 10 mW comes from a chargecurrent of 2.5 mA into a 4V battery. The time ratio of 20 us to 100 msis the same ratio of 2 uW input power to 10 mW output power thusallowing the average power harvested to be 2 uW. Pulse harvesting inthis manner can be performed if the boost converter circuit is poweredfrom the battery 140 at a lower power level than is being transferredfrom the capacitor C3 to the battery 140. Since the energy istransferred as a burst from the capacitor C3 to the battery 140, theamount of power used during the short bust that is consumed by the boostconverter circuit can be much greater than the instantaneous powergenerated by the low voltage source 122. In an exemplary application,the low voltage source 120 generates about 2 uW of average power. Theboost converter circuit supply power can be much greater than 2 uW, forexample 40 uW, because the pulse energy transfer from the capacitor C3to the battery 140 is delivered in milliamps, 2.5 mA in this example.During pulse harvesting period when the boost converter circuit isturned on, the power used to operate the boost converter circuit is lessthan the amount of power delivered to the battery.

In an exemplary implementation, the low voltage source 120 generatesabout 2 uW at about 0.5V. The capacitor C3 has a capacitance of about 10uF, the capacitor C4 has a capacitance of about 0.1 uF, and the inductorhas an impedance of about 1 uH. When compared to the circuit 10 of FIG.1A, the energy harvesting circuit 100 uses an inductor L2 having animpedance in microhenries instead of millihenries as with the inductorL1 (FIG. 1A). The battery 140 has a battery voltage of about 4V, and theboost controller 130 boosts the voltage of the capacitor C3 to about4.2V. It is understood that the energy harvesting circuit 100 can beconfigured with components having alternative values. For example, thelow voltage source 120 can be configured to generate voltage down tomuch lower levels then the exemplary 0.5V, such as down to 10 mV. Theenabling circuit is simply changed to have a lower input referencevoltage.

In operation, the energy generated by the low voltage source 120 chargesthe capacitor C3 while the boost converter circuit is turned off. Thevoltage VC of the capacitor C3 is monitored by the comparator 150 andcompared to the reference voltage Vref. While capacitor voltage VC isless than the reference voltage Vref, the PFM Enable signal output fromthe comparator 150 to the boost controller 130 is low and the boostconverter circuit is turned off. Once the capacitor voltage VC isgreater than or equal to the reference voltage Vref, the PFM Enablesignal goes high, thereby turning on the boost converter circuit. Whenthe boost converter circuit is turned on, the energy stored in thecapacitor C3 is discharged from the capacitor C3 and boosted by theboost converter circuit to a voltage that exceeds the battery voltage.The boosted voltage charges the battery 140. The capacitor C3 dischargesuntil the capacitor voltage VC drops to a minimum voltage Vmin, at whichpoint the PFM Enable signal goes low, thereby turning off the boostconverter circuit. The low voltage source 120 charges the capacitor C3while the boost converter circuit is turned off.

FIG. 4 illustrates an exemplary capacitor voltage function for thestorage capacitor C3 in FIG. 2. Portions 162 and 166 of the voltagefunction corresponds to the discharging of the capacitor C3. Thecapacitor C3 starts discharging once the capacitor voltage VC reachesthe reference voltage Vref, and continues to discharge until thecapacitor voltage VC drops to the minimum voltage Vmin that is set to beefficient. Portion 164 of the voltage function corresponds to thecharging of the capacitor C3. The capacitor C3 starts charging once thecapacitor voltage VC reaches the minimum voltage Vmin, and continues tocharge until the capacitor voltage increases to the reference voltageVref. The capacitor C3 charges and discharges according to differenttime constants, as indicated by the differently shaped curves of theportions 162 (discharging) and 164 (charging). The capacitor C3discharges according to a discharge time constant (TC) determined by theimpedance of the inductor L2 and the capacitor C3 (TC=ZL*C3). Thecapacitor C3 charges according to a charging TC determined by theinternal resistance RS of the low voltage source 120 and the impedanceof the capacitor C3 (TC=RS*C3). In implementation, the ratio of thedischarge time constant to the charge time constant is configured to behundreds or thousands to one. The charge time constant is relativelylarge because the low voltage source 120 has an impedance RS that ismuch larger than an impedance ZL of the boost converter. The dischargetime constant is relatively low because energy is delivered from thecapacitor to the battery 140 as a high energy pulse. For illustrativepurposes, the voltage function shown in FIG. 4 shows the generalcharging and discharging nature of the capacitor C3. However, thedischarge time constant manifested in curves 162 and 166, and the chargetime constant manifested in curve 164 are not representative of animplementation where the ratio is hundreds or thousands to one. Thecurves shown in FIG. 4 are merely a convenience so that a full cycle ofcharging and discharging can be shown on the same graph.

Embodiments of the energy harvesting circuits above are described as avoltage source that provides a DC input to the storage capacitor. Theenergy harvesting circuits can be alternatively configured to include anAC power source, in which case a rectifying circuit is added between theenergy source and the storage capacitor

Although the energy harvesting circuit is described as harvesting energyto charge a battery, the energy harvesting circuit can be used to chargeother types of storage elements, such as super capacitors. In general,any storage element can be used that has the capability of acceptingpulse charges and integrating these charges as an accumulated energyreserve. The energy harvesting circuit provides means for charging thestorage element using an energy source having a low power level relativeto the storage element energy storage capacity.

FIG. 2 shows the comparator 150, the boost controller 130, thetransistor T2, the inductor L2, the diode D2, and the capacitor C4 asseparate components. In some embodiments, the comparator 150 isintegrated with the boost controller 130. In some embodiments, thetransistor T2 is integrated with the boost controller 130. In someembodiments, the diode D2 is integrated with the boost controller 130,and the capacitor C4 and the inductor L2 are external. In some cases,the inductor L2 can be integrated into the boost controller, for examplewhere the inductor L2 has an impedance of 1 uH, and/or the capacitor C4can be integrated into the boost controller, for example where thecapacitor C4 has a capacitance of 0.1 uF. In general, some or all ofthese components can be integrated.

Although the embodiments of the energy harvesting circuit describe anenergy harvesting circuit having a boost converter, it is understoodthat the principles described can be applied to alternative types ofstep-up converters or to step-down converters.

The energy harvesting circuit has been described in terms of specificembodiments incorporating details to facilitate the understanding of theprinciples of construction and operation of the energy harvestingcircuit. Such references, herein, to specific embodiments and detailsthereof are not intended to limit the scope of the claims appendedhereto. It will be apparent to those skilled in the art thatmodifications can be made in the embodiments chosen for illustrationwithout departing from the spirit and scope of the energy harvestingcircuit.

What is claimed is:
 1. A circuit comprising: a. an energy storage element; b. an energy source; c. a storage capacitor coupled to the energy source and configured to store energy output from the energy source; d. a power converter circuit coupled between the storage capacitor and the energy storage element, wherein the power converter circuit converts a storage capacitor voltage stored by the storage capacitor to a voltage to charge the energy storage element; and e. an enabling circuit coupled between the energy source and the power converter circuit, wherein the enabling circuit pulse modulates the power converter circuit to enable burst energy transfer from the storage capacitor to the energy storage element whenever the storage capacitor voltage reaches a reference voltage, further wherein the enabling circuit is powered by the energy source.
 2. The circuit of claim 1 wherein the enabling circuit is configured to compare the storage capacitor voltage to the reference voltage and output an enabling signal to the power converter circuit if the capacitor voltage is greater than or equal to the reference voltage and output a disabling signal to the power converter circuit if the capacitor voltage is less than the reference voltage.
 3. The circuit of claim 2 wherein the power converter circuit is configured to enable burst energy transfer from the storage capacitor to the energy storage element when turned on by the enabling signal and to disable energy transfer when turned off by the disabling signal.
 4. The circuit of claim 2 wherein when the power converter circuit is disabled there is no power drain on the energy source due to a load.
 5. The circuit of claim 2 wherein when the power converter circuit is disabled the only power drain on the energy source is due to parasitic drain.
 6. The circuit of claim 1 wherein the energy storage element has a storage element voltage and the energy source generates power at a source voltage, further wherein the source voltage is less than the storage element voltage.
 7. The circuit of claim 1 wherein the energy source generates power in the range of about 2 uW to about 200 uW.
 8. The circuit of claim 7 wherein the energy source generates between about 0.4V and about 0.8V.
 9. The circuit of claim 1 wherein the power converter circuit comprises a step-up converter.
 10. The circuit of claim 1 wherein the power converter circuit comprises a step-down converter.
 11. The circuit of claim 1 wherein the energy storage element comprises a battery.
 12. The circuit of claim 1 wherein the energy storage element comprises a super capacitor.
 13. The circuit of claim 1 wherein the power converter circuit is powered by the energy storage element when the power converter circuit is turned on.
 14. The circuit of claim 1 wherein the power converter circuit comprises a converter controller circuit and a transistor coupled to the converter controller circuit, the transistor configured to enable and disable current flow between the storage capacitor and the energy storage element, and the converter controller circuit is configured to turn on and off the transistor.
 15. The circuit of claim 1 further comprising a reference voltage supply coupled to the enabling circuit and configured to supply the reference voltage to the enabling circuit, wherein the reference voltage supply is powered by the energy source.
 16. The circuit of claim 1 wherein the energy source comprises a DC power source.
 17. The circuit of claim 1 wherein the energy source comprises an AC power source.
 18. The circuit of claim 17 further comprising a rectifying circuit coupled between the AC power source and the storage capacitor.
 19. The circuit of claim 1 wherein the enabling circuit has positive feedback.
 20. A circuit comprising: a. an energy storage element; b. an energy source; c. a storage capacitor coupled to the energy source and configured to store energy output from the energy source; d. a power converter circuit coupled between the storage capacitor and the energy storage element, wherein the power converter enables energy transfer from the storage capacitor to the energy storage element when turned on by an enabling signal and disables energy transfer when turned off by a disabling signal; and e. an enabling circuit coupled between the energy source and the power converter circuit, wherein the enabling circuit is powered by the energy source, further wherein the enabling circuit compares a storage capacitor voltage to a reference voltage and outputs the enabling signal to the power converter circuit if the capacitor voltage is greater than or equal to the reference voltage and outputs the disabling signal to the power converter circuit if the capacitor voltage is less than the reference voltage.
 21. The circuit of claim 20 wherein when the power converter circuit is disabled there is no power drain on the energy source due to a load.
 22. The circuit of claim 20 wherein when the power converter circuit is disabled the only power drain on the energy source is due to parasitic drain.
 23. The circuit of claim 20 wherein the energy storage element has a storage element voltage and the energy source generates power at a source voltage, further wherein the source voltage is less than the storage element voltage.
 24. The circuit of claim 20 wherein the energy source generates power in the range of about 2 uW to about 200 uW.
 25. The circuit of claim 24 wherein the energy source generates between about 0.4V and about 0.8V.
 26. The circuit of claim 20 wherein the power converter circuit comprises a step-up converter.
 27. The circuit of claim 20 wherein the power converter circuit comprises a step-down converter.
 28. The circuit of claim 20 wherein the storage element comprises a battery.
 29. The circuit of claim 20 wherein the storage element comprises a super capacitor.
 30. The circuit of claim 20 wherein the power converter circuit is powered by the energy storage element when the power converter circuit is turned on.
 31. The circuit of claim 20 wherein the power converter circuit comprises a converter controller circuit and a transistor coupled to the converter controller circuit, the transistor configured to enable and disable current flow between the storage capacitor and the energy storage element, and the converter controller circuit is configured to turn on and off the transistor.
 32. The circuit of claim 20 further comprising a reference voltage supply coupled to the enabling circuit and configured to supply the reference voltage to the enabling circuit, wherein the reference voltage supply is powered by the energy source.
 33. The circuit of claim 20 wherein the energy source comprises a DC power source.
 34. The circuit of claim 20 wherein the energy source comprises an AC power source.
 35. The circuit of claim 34 further comprising a rectifying circuit coupled between the AC power source and the storage capacitor.
 36. The circuit of claim 20 wherein the enabling circuit has positive feedback.
 37. A circuit comprising: a. an energy storage element; b. an energy source; c. a storage capacitor coupled to the energy source and configured to store energy output from the energy source; d. a power converter circuit coupled between the storage capacitor and the energy storage element, wherein the power converter enables energy transfer from the storage capacitor to the energy storage element when turned on by an enabling signal and disables energy transfer when turned off by a disabling signal; e. an enabling circuit coupled between the energy source and the power converter circuit, wherein the enabling circuit is powered by the energy source, further wherein the enabling circuit compares a storage capacitor voltage to a reference voltage and outputs the enabling signal to the power converter circuit if the capacitor voltage is greater than or equal to the reference voltage; and f. a disabling circuit coupled between the energy source and the power converter circuit, wherein the disabling circuit is powered by the energy storage element only when the power converter circuit is enabled, further wherein the disabling circuit compares the storage capacitor voltage to the reference voltage and outputs the disabling signal to the power converter circuit if the capacitor voltage is less than the reference voltage. 